Expose dx and dy on the DragEvent. about d3-drag

[mbostock]

Hmm. That documentation is actually incorrect; event.dx and event.dy represent the change in coordinates relative to the previous drag (or start) event, not the start of the drag gesture. (See this example, which shows how event.dx and event.dy are used to increment the x and y attributes of a rect on each drag event.)

But, incorrect documentation aside, it wouldn’t be hard to also expose the change in position since the previous event. So, okay.

[altocumulus]

Thank you, very helpful! This paved the way for my approach to "Attaching drag behaviour without using data().enter() in D3 v4.0.0-alpha.40" on SO.